Resonant tunneling diodes (RTDs) exhibit the highest oscillation frequency of any known semiconductor device, for example in excess of 720 GHz. Further, the voltage-current characteristic of an RTD has an operating region in which the RTD effectively exhibits a negative resistance characteristic. In this regard, a positive resistance consumes power, and thus will attenuate an electrical signal. Conversely, a negative resistance essentially represents gain, and is useful in implementing circuits such as an oscillator or an amplifier.
A problem is that, when an RTD is biased to operate at a selected frequency, it usually has a tendency to oscillate at some other undesired frequency, for example due to a parasitic characteristic, such as the inductance in a lead extending to a battery that powers the circuit. One known solution to this instability is to couple a resistor in parallel with the RTD. This approach provides increased stability at frequencies other than the operating frequency. However, at the operating frequency, the positive resistance of the external resistor tends to counteract the negative resistance of the RTD, and thus also tends to counteract the gain characteristic of the RTD. Consequently, the use of an external resistor in this manner is counterproductive to the goal of efficiently exploiting the gain characteristic of the RTD.
A different known approach for stabilizing an RTD is to couple the two conductors at one end of a transmission line to opposite ends of the RTD, and to couple an inductive/capacitive (LC) resonant circuit to the other end of the transmission line. However, for many applications, the use of a transmission line is cumbersome and impractical. This can be true even when the transmission line is implemented as part of the integrated circuit that contains the RTD.
Another consideration is that the output impedance of an RTD drops to a very low value at high operational frequencies. Also, some applications require a power-handling capacity which is in excess of the power-handling capacity of a single RTD. It is possible to couple several RTDs in series, in order to obtain a larger overall impedance and a larger overall power-handling capacity. However, this presents an even greater potential problem for direct current (DC) instability, if the DC bias across the entire string is not divided relatively uniformly among the individual RTDs. A respective resistor can be coupled in parallel with each RTD in the string, in the manner discussed above. These resistors will provide increased stability, and ensure that the DC bias is uniformly allocated among the individual RTDs. However, as also discussed above, the positive resistance of each external resistor will at least partially counteract the negative resistance and associated gain of each RTD at the intended operational frequency, and will thus tend to undermine efforts to optimally and efficiently harness the gain characteristic.
As another alternative, and as noted above, it would theoretically be possible to couple each RTD in the string to a respective different transmission line with a respective resonant circuit. However, where there are several RTDs coupled in series, it is highly impractical to provide a separate transmission line and resonant circuit for each RTD.